This invention relates to a transistorized high-frequency power amplifier, which may be for a UHF television satellite transmission station.
As a consequence of technical progress in high-frequency high-power transistors, approaches have been made to realizing a high-frequency power amplifier with a solid-state circuit. By way of example, a solid-state UHF power amplifier comprising a dozen or more transistors, each having a saturated output of the order of 20 watts, has been developed for a UHF television satellite transmission station of the output power of several hundred watts.
The high-frequency high-power transistors, however, have in general a very low input impedance. It is therefore difficult to realize a wide-band matching circuit for the transistor. For example, the risistance component of the input impedance is generally only a few ohms or less. As a result, even a small reactance component may inevitably render the Q value of the matching circuit unduly high. As a further result, a solid-state high-frequency power amplifier has but a restricted operable bandwidth.
On the other hand, the matching circuit must have a large impedance transformation or conversion ratio due to the small input impedance of the transistor. As a result, use has to be made either of a line of a very small impedance or a capacitor of a very large capacitance. It has, however, been impossible to form such a matching circuit by conventional integrated circuit techniques, such as hybrid integrated circuit techniques, wherein use is made of a microstrip line on an alumina or a similar dielectric layer and a chip capacitor. In particular, the wavelength of a UHF signal in the microstrip line is as long as several scores of centimeters. This makes it necessary to resort at least partly to a lumped constant circuit element. A thin-film integrated circuit element is also insufficient to achieve a desired circuit constant and desired high-frequency characteristics. Moreover, it has been impossible to reduce the high-frequency loss of the dielectrics with a view to suppressing heat generation in the high-frequency circuit and to reduce the temperature dependency of the dielectric constant with a view to suppressing changes caused in the circuit characteristics by a transistor or a like active circuit element put in operation. A conventional transistorized high-frequency high-power amplifier has therefore been bulky, expensive, and yet unreliable.